The present invention relates to a method of controlling a permanent magnet alternating current motor.
The torque of an alternating-current motor is generated by means of an armature circuit, normally the stator, a supply voltage and an excitation circuit, normally the rotor. The supply voltage is fed into the stator winding, which is fitted in stator slots. Excitation is developed by means of permanent magnets or an excitation winding fitted in the rotor. The steadiness of the torque depends on how closely the field developed in the air gap of the motor approaches the sinusoidal form. A curve form differing from the sinusoidal form produces torque variations at a frequency determined by the harmonic components contained in it, giving rise to mechanical vibration and noise.
In addition to the supply voltage, the form of the air gap field depends on the form of the field generated by the excitation, among other things. Moreover, in motors having a small number of slots per phase and pole, a notable problem is the so-called slot-harmonic field generated by the current flowing in each slot. For instance, in a three-phase motor with one slot per phase, a six-fold motor supply frequency is problematic.
Public patent application WO 98/26643 presents a solution in which the moment resulting from the harmonic voltage is eliminated by adding to the motor supply voltage a harmonic that will compensate the disturbing moment produced by the slot harmonic. In this solution, the harmonic voltage added to the supply voltage produces a moment that is equal to the disturbing moment ripple but in opposite phase. In practice, however, as regards motor regulation, the solution described in the publication is difficult to implement and the apparatus comprises a large number of components.
The object of the present invention is to achieve a solution that allows simple regulation without superfluous voltage conversions while at the same time minimising the required measuring and control circuits.
According to the invention, harmonic moments of the motor are effectively eliminated by adding to the current reference an auxiliary quantity that will produce a moment in opposite phase with the disturbing moment of the motor.
According to an embodiment of the invention, the saturation of the current regulator is compensated by feeding the third harmonic of the current into the current regulator.
For each elevator, the required phase shifts and amplitudes are determined e.g. experimentally.